This invention relates to aluminum alloy products having superplastic properties, and to methods of preparing such products.
Superplastic alloys are characterized by elevated-temperature forming properties somewhat similar to those of plastics and glass. That is to say, at temperatures within a superplastic forming range (determined by the composition of the alloy), these alloys are able to undergo extensive deformation under small forces without fracture or failure by necking or center voids; for instance, unlike ordinary sheet metal, superplastic alloy sheet at forming temperatures can be formed into complex shapes by blow molding with compressed air at relatively low pressure. Two criteria currently applied to define superplasticity are the properties (at forming temperatures) of tensile elongation of at least 200% and strain rate sensitivity index m of at least 0.3, although alloys which attain somewhat lesser values (e.g. elongation of at least about 100%) may nevertheless be usefully superplastic for many purposes, and will be understood to be included within the term "superplastic alloys" as used herein.
Several superplastic alloys are known, and have been found to have utility in making metal parts of configurations difficult to produce by conventional techniques. One such alloy is a zinc-based alloy containing 22% aluminum (all percentages herein being expressed as percent by weight unless otherwise specifically indicated). Another, an aluminum-based alloy containing 6% copper and 0.5% zirconium, is advantageous for various applications in that it is lighter in weight, and has better creep resistance and surface finish, than the zinc-based alloy, but it is relatively difficult to produce and somewhat susceptible to corrosion. The binary eutectic alloy of aluminum with 7.6% calcium is also superplastic, but cannot readily be cold-worked owing to its brittleness.